DC power for example battery or solar power is often used in low-voltage applications and/or as power supply for electric circuits. A DC switching device is provided between the DC power and a load of an electric circuit system to connect and disconnect the DC power to the load.
In a working position, the movable contact is in contact with the stationary contact, while when a current is disconnected, an air gap is established and an arc is inevitably generated between the contacts. This arc can be very destructive and must therefore be contained, cooled and extinguished in a controlled way so that the air gap between the contacts can withstand the voltage in the circuit. One way to withstand the voltage is to provide the DC switching device with an arc-extinguishing chamber including one or more splitter plates for dividing the arc into partial arcs and thereafter cooling it off.
Voltage is a more important factor for a DC switching device when breaking a current. For an AC switching device, current zero is used to facilitate arc extinguishing since at current zero it is optimal for preventing an arc from continuing. Nevertheless, this is not a case for the DC application which the current has a steady state value.
Left uncontrolled, voltage may give a new life to the arc. If it reignites, it can damage the whole electrical system. Therefore, the DC switching device has to take this process into account by opening the contacts and extinguishing the arc simultaneously.
A high breaking capability is one of desired features when designing/selecting a DC switching device. However, the short distance of the air gap between the contacts results in a limited voltage drop across the air gap, the breaking capability of the DC switching device in term of voltage, current and time is therefore limited when the DC switching device is used in a relatively higher voltage and current application.
Many switching device manufactures produce AC switching devices including three poles corresponding to three AC phases and a contacting unit is provided for each of poles. When this type of AC switching devices is adapted for a DC application, a simple wiring is obtained by connecting the contacting units to each other in series. This feature is often desired by many customers.
A U.S. Pat. No. 5,004,874 presents a DC switching apparatus having two arc extinguishing chambers each comprising a pair of spaced conductors providing cooperable arc runners divergent toward a row of non-ferromagnetic splitter plates and a stationary contact conductively mounted on one conductor, the stationary contacts of respective chambers being mounted on respectively opposite conductors and corresponding conductors in respective chambers are conductively connected to each other and to power terminals of the apparatus, permanent magnets applying a magnetic field across the respective chamber for moving an arc within the chamber, ferromagnetic plates providing flux return paths to optimize and maximize the magnetic field, a movable contact extending into each chamber bridging the stationary contacts and movable to separate from the stationary contacts, drawing an arc therebetween in each chamber, the arc in one chamber bridging the pair of conductors within that chamber establishing a circuit comprising the arc between the conductors and the power terminals in shunt of the movable contact, thereby eliminating the arc in the other chamber, the bridging arc being extinguished in the splitter plates, interrupting the circuit. The magnetic fields are applied in opposite directions in the respective chambers for non-polarized operability of the apparatus and are distorted within the splitter plate area to drive and maintain an arc at a stable arc position against a thickened sidewall portion to withstand erosion. The arrangement of two arc-extinguishing chambers enables the DC switching device to be used as a non-polarized switching device.